The CDC2 kinase associated with its cyclin partners has been shown to play an important role during G2/M progression in eukaryotic cells. However, recent studies demonstrate that activation of the CDC2 kinase itself is not sufficient to trigger mitosis in some eukaryotic cells such as those in Saccharomyces cerevisiae (Amon, et al., Nature, 355:368, 1992; Sorger and Murray, Nature, 355:365, 1992; Stueland et al., Mol. Cell. Bio., 13:3744, 1993) and Aspergillus nidulans (Osmani, et al., Cell, 67:283, 1991a). Furthermore, detailed analysis of mouse oocyte maturation reveals that CDC2 histone H1 kinase activity does not increase during the G2/M transition as indicated by germinal vesicle breakdown (GVBD) (Choi, et al., Development, 113:789, 1991; Jung, et al., Int. J Dev. Biol., 37:595, 1993; Gavin, et al., J Cell Sci., 107:275, 1994). These results suggest that there might be other mitotic activation pathway(s) remaining to be identified.
Recent studies have identified a novel mitotic kinase, NIMA, encoded by the Aspergillus nimA gene (Osmani, et al., Cell, 53:237, 1988). NIMA kinase activity is tightly regulated during the nuclear division cycle, peaking in late G2 and M. Overexpression of NIMA promotes entry of Aspergillus cells into M (Osmani, et al., Cell, 53:237, 1988; Lu and Means, EMBO J., 13:2103, 1994). Thus, NIMA is important for progression into mitosis in Aspergillus.
NIMA is a protein-Ser/Thr kinase, biochemically distinct from other protein kinases, and its phosphotransferase activity is regulated by Ser/Thr phosphorylation. It has recently been shown that the NIMA mitotic pathway is not restricted to Aspergillus, but also exists in vertebrate cells (Lu and Hunter, Cell, 81:413, 1995a). In Xenopus oocytes, NIMA induces germinal vesicle breakdown without activating Mos, CDC2 or MAP kinase. In HeLa cells, NIMA induces mitotic events without activating CDC2, whereas dominant-negative NIMA mutants cause a specific G2 arrest. In addition, O'Connell, et al (EMBO J, 13:4926, 1994) have also demonstrated that NIMA induce premature chromatin condensation in fission yeast and HeLa cells. These results reveal the existence of a NIMA-like mitotic pathway in other eukaryotic cells.
Peptidyl-prolyl cis/trans isomerases (PPIases, proline isomerases) are ubiquitously expressed enzymes catalyzing the cis/trans isomerization of the peptidyl-prolyl peptide bond which can be the rate-limiting step in protein folding or assembly under some circumstances. Cyclophilins and FK506-binding proteins (FKBPs) are two well characterized families of PPIases that share little if any amino acid similarity to each other. However, the members of each family contain a core structure that has been highly conserved from prokaryotes to eukaryotes. The importance of these PPIases is highlighted by the findings that cyclophilin and FK506-binding proteins are the targets of immunosuppressive drugs cyclosporin A and FK506, respectively, and play an important role in cell signaling in T cell activation, although none of these genes have been shown to be essential for life (for review see Schreiber, Science, 251:283, 1991; Fruman, et al., FASEB J, 8:391, 1994).
The recent discovery of parvulin has led to the identification of a third family of PPIases, which show little homology with either cyclophilins or FKBPs and is not sensitive to the immunosuppressive drugs (Rahfeld, et al., FEBS Lett., 352:180, 1994a and FEBS Lett., 343:65, 1994b). A sequence homology search identified several other members of this family including those involved in protein maturation and/or transport and the ESS1 gene (Rudd, et al., TIBS, 20:12, 1995). ESS1 is an essential gene for the growth in budding yeast and previous results suggested that it may be required at later stages of the cell cycle (Hanes, et al., Yeast, 5:55, 1989). ESS1 was recently reisolated as PTF1 in a screen for genes involved in mRNA 3' end maturation. Ptf1 was shown to contain a putative PPIase domain, but PPIase activity could not be demonstrated (Hani, et al., FEBS Lett., 365:198, 1995). So far, none of the PPIases have been shown to be specifically involved in cell cycle control.
There is a need to identify components of the mammalian NIMA mitotic pathway in order to identify genes essential for life. Identification of such genes has several utilities including the identification of appropriate therapeutic targets, candidate genes for gene therapy (e.g., gene replacement), mapping locations of disease-associated genes, and for the identification of diagnostic and prognostic indicator genes, for example.